Hydration effects on the microwave dielectricity in dry poly(dA)-poly(dT) DNA

Using a microwave cavity-perturbation method, the temperature dependence of the complex dielectric constant (∈ = ∈₁ + i∈₂) at 16.3 GHz is measured on a dry poly(dA)-poly(dT) DNA sample prepared under three-different relative humidity (RH) of 0, 11 and 93%. For RH = 0%, the temperature dependence of both ∈₁ and ∈₂ are explained by the rotational motion of water molecules electrostatically bonding to the negatively charged phosphate group in the primary hydration shell. The activation-type behavior in ∈₂ is discussed with a double-well potential for a process that the electric dipole of the hydrated water is thermally rotated to the adjacent phosphate groups. For RH = 11%, ∈₂ takes a broad maximum at 280 K due to the collective motion of water molecules in the primary hydration shell. While these features qualitatively resemble to those of free water, the absolute values are much small due to an extremely small number of hydrated water molecules. Moreover the conformation is expected to be of A form, in which the hydration bridge could be constructed in the major groove via hydrogen bonds as suggested by previous X-ray and neutron diffraction experiments. For RH = 93%, in addition to the collective motion of the primary hydration shell, a freezing process of the secondary hydration shell appears. Independently on RH, the hydrated water molecules at low temperatures around 200 K or below are hard to respond to microwave electric fields, and hence both the rotational and collective motions cannot be permitted. Approaching to 420 K, ∈₂ becomes suppressed converging to the same value. The suppression comes from a thermally induced dehydration resulting in eventual reduction of the hydration density down to 0.1 water molecules per nucleotide as revealed by our previous infrared spectroscopy.